Methods and Apparatus in a Wireless Communication System

ABSTRACT

The present invention relates to methods and apparatus in a RBS and a UE for reference signal (RS) measurements in an OFDM system, that enable having a configurable RS transmission bandwidth which is smaller than the system bandwidth. This allows for better interference coordination of RS, which in turn improves the UE RS measurements used for different services such as positioning. The RBS retrieves the RS transmission bandwidth, determines a RS measurement bandwidth based on this RS transmission bandwidth, and transmits the determined bandwidth to the UE. The UE receives the RS measurement bandwidth and measures the RS in a bandwidth determined based on the received measurement bandwidth and the UE capability.

RELATED APPLICATIONS

This application is a continuation application of the U.S. patentapplication with Ser. No. 14/325,555, filed on Jul. 8, 2014 and entitled“Methods and Apparatus in a Wireless Communication System”, whichapplication is a continuation application of the U.S. patent applicationwith Ser. No. 12/717,222, filed on Mar. 4, 2010 and entitled “Methodsand Apparatus in a Wireless Communication System,” which claims priorityunder 35 U.S.C. §119(e) from the U.S. Provisional Patent Applicationwith Ser. No. 61/172,911, filed on Apr. 27, 2009, and entitled “FlexibleBandwidth Configuration Facilitating Positioning Measurements in LTE,”and from the U.S. Provisional Patent Application with Ser. No.61/173,357, filed on Apr. 28, 2009, and entitled “Flexible BandwidthConfiguration Facilitating Positioning Measurements in LTE,” and alsoclaims priority under 35 U.S.C. §365(c) from the International PatentApplication with application number PCT/SE2009/050951, filed on Aug. 25,2009, and entitled “Methods and Arrangements in a WirelessCommunications System”, each of which applications is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention relates to reference signal measurements in anOFDM system and in particular to a radio base station and a userequipment, and to a method for reference signal measurements used fore.g. positioning.

BACKGROUND

The Universal Mobile Telecommunication System (UMTS) is one of the thirdgeneration mobile communication technologies designed to succeed GSM.3GPP Long Term Evolution (LTE) is a project within the 3^(rd) GenerationPartnership Project (3GPP) to improve the UMTS standard to cope withfuture requirements in terms of improved services such as higher datarates, improved efficiency, lowered costs etc. The Universal TerrestrialRadio Access Network (UTRAN) is the radio access network of a UMTSsystem and evolved UTRAN (E-UTRAN) is the radio access network of an LTEsystem. As illustrated in FIG. 1, an E-UTRAN typically comprises userequipments (UE) 150 wirelessly connected to radio base stations (RBS)100, commonly referred to as eNodeB. The eNodeB serves one or more areasreferred to as cells 110.

Mobile user positioning is the process of determining UE coordinates inspace. Once the coordinates are available, the position can be mapped toa certain place or location. The mapping function and the delivery ofthe location information on request are parts of the location servicewhich is required for the basic emergency services. Services thatfurther exploit the location knowledge or that are based on locationknowledge to offer customers some additional value, are referred to aslocation-aware and location-based services, respectively.

There exist a variety of positioning techniques in wirelesscommunications networks, differing in their accuracy, implementationcost, complexity, applicability in different environments, etc. Inexisting networks, the most common are UE assisted solutions where aserving mobile location center 120 (SMLC in GSM and UMTS, enhanced SMLC(eSMLC) in LTE) calculates the UE position based on measurementsreported by the UE. The SMLC/eSMLC 120 is either a separate networkelement (as illustrated in FIG. 1) or an integrated functionality in theRBS. Among such methods, Assisted Global Positioning System (A-GPS)typically provides the best accuracy. Combining the mobile technologyand GPS, A-GPS enhances the UE receiver sensitivity by providing orbitand other data to the UE. Drawbacks of A-GPS is that a GPS-equipped UEis required, and that it doesn't function in certain environments suchas tunnels, indoor areas and dense urban areas. Therefore othercomplementing methods for positioning are needed. These methods usemeasurements of the time difference of arrival (TDOA) of signals betweenthe cellular antenna and the UE. In UMTS observed TDOA (OTDOA) is used.In GSM a variant called Enhanced Observed Time Difference (E-OTD) isused.

The technique currently adopted for LTE-based positioning is OTDOA.OTDOA is a multi-lateration based technique estimating TDOA of signalsreceived from three or more sites. To enable positioning, the UE shouldbe able to detect signals from at least three geographically dispersedRBS. This implies that the signals need to have high enoughsignal-to-interference ratios (SINR). Furthermore, the signals need tobe transmitted frequently enough to meet the service delay requirements.In order to meet the accuracy requirements, the signals may need to beaccumulated over multiple sub frames.

There is currently no completely standardized positioning method for LTEand therefore there is no existing reference solution. To enablepositioning measurements in LTE, a straightforward solution would be tomeasure standardized signals that are always transmitted from LTE RBS,e.g. synchronization signals (SS) or cell-specific reference signals(RS). SS and cell-specific RS (CRS) are physical signals used to supportphysical-layer functionality and they do not carry any information fromthe Medium Access Control (MAC) layer. Both signals are transmittedaccording to a pre-defined pattern, i.e. in selected subcarriers andtime slots, and the pattern is typically relatively sparse.

In LTE, SS are transmitted in downlink and are primarily used in thecell search procedure, i.e. for the UE to identify a cell andsynchronize to it in downlink in order to read the broadcast channelinformation. As shown in FIG. 2a , SS are transmitted in sub frame 0,220, and sub frame 5, 230, of a radio frame 210. A SS consists ofPrimary SS (PSS) 240 and Secondary SS (SSS) 250. First, a cell identityis read from PSS, and then the cell identity group is read from SSS. Thecell identity can then be used to determine the CRS sequence and itsallocation in the time-frequency grid. In FIG. 2b , it is shown that theSS occupy 62 resource elements in the center of the allocated bandwidth.

CRS are transmitted over the entire system bandwidth and in every subframe, i.e. more frequently than SS. In normal sub frames with a normalcyclic prefix where each time slot comprises seven OFDM symbols, CRS aretransmitted on the resource elements (RE) shown in FIG. 3a ,illustrating the RE 310 of a time-frequency resource grid for one subframe 311 in time and 12 subcarriers 312 in frequency (the number ofsubcarriers corresponding to a physical resource block (PRB)). FIG. 3ashows the RE used for CRS 313 in a system with a single transmitantenna. In such a system, up to six different shifts in frequency(frequency reuse factor =6) and 504 different signals can be used forthe CRS. With two transmit antennas, the maximum frequency reuse factorreduces to three, which is shown in FIGS. 3b and 3c . FIG. 3billustrates the time frequency resource grid for a first antenna port,indicating the RE used for CRS for this first antenna port 313 (similarto FIG. 3a ) as well as the RE reserved for CRS for the second antennaport 314. FIG. 3c illustrates the time frequency resource grid for thesecond antenna port, indicating the RE used for CRS for this secondantenna port 316, corresponding to the reserved RE 314 in FIG. 3b , aswell as the RE reserved for CRS for the first antenna port 315. Withfour transmit antennas, the possibilities are even more limited as shownin FIG. 3d , illustrating the time frequency resource grid for a firstantenna port out of four. In FIG. 3d , the RE used for CRS for thisfirst antenna port 317 as well as the RE reserved for CRS for the otherthree antenna ports 318 are indicated. Other CRS patterns are definedfor sub frames with extended cyclic prefix and for multicast broadcastsingle frequency network (MBSFN) sub frames.

However, it has been shown that using SS and CRS for positioning withoutinterference management would result in positioning coverage problemsdue to low SINR and/or insufficient number of strong signals fromdifferent RBS. The problem is particularly relevant for synchronizednetworks or networks with high data load, as there is a high probabilityof parallel transmissions in multiple cells on the RE used for CRS or SSwhich leads to high interference. Furthermore, the SS transmissionfrequency is not sufficient for the positioning requirements.

To improve positioning measurements and address the hearability problem,it has been proposed in 3GPP to introduce positioning RS (PRS), whichcould be designed according to transmission patterns characterized by alower collision probability. The transmission periodicity for PRS isunder discussion. In general, PRS may or may not be transmitted inmultiple consecutive sub frames and the periodicity can be configuredstatically or semi-statically.

In respect to the frequency dimension, given a PRS transmission patternper PRB, the simplest solution would be to repeat the same pattern inall PRB of the same sub frame, i.e., over the entire bandwidth.Transmitting PRS over a large bandwidth generally improves positioningaccuracy due to a higher measurement resolution and a lower probabilityof being in unfavourable frequency-selective fading conditions. Thedrawback is that a large bandwidth gives a high UE complexity.Furthermore, a smaller bandwidth may be sufficient to achieve therequired accuracy, and using the entire bandwidth is then a waste ofresources.

At a high system load, there is no gain in introducing the new PRSwithout interference coordination. One of the approaches for reducinginterference is to transmit PRS during low-interference sub frames (LIS)in which PDSCH transmissions are suppressed. FIG. 4 illustrates anexample of a LIS 400 for a single cell with a possible PRS pattern,where the data transmission is suppressed in all PRB 440 of the entiretransmission bandwidth of the cell 450. In the LIS 400, there are REused for PRS 410, RE used for control signalling 420, but the rest ofthe REs are free from data transmission 430. To achieve an even higherinterference reduction, LIS can be aligned among the cells. For the LISalignment, inter-cell coordination may or may not be needed, dependinge.g. on if LIS occurrences are configured statically or dynamically.FIG. 5 illustrates an example with aligned LIS 500 for a synchronizednetwork with three-cell sites and a frequency reuse of three for thePRS. In the frequency dimension, only one PRB per cell is illustrated.The RE used for PRS in the current cell 520 are indicated in each cell'stime-frequency resource grid, together with the RE used for PRS in someother cell 530. Cells within the same re-use group (e.g. cell(1,1) andcell(2,1)) will have colliding PRS 510.

In LTE networks, some sub frames can be configured to be MBSFN subframes. Such sub frames are utilized for multicast/broadcasttransmissions such as Mobile TV, and will not be utilized initially whenthe service is not supported. These sub frames could thus also beconsidered as low-interference sub frames during which transmission ofPRS would be allowed. This is only a feasible solution in the releasewhich does not support Multimedia Broadcast Multicast Service (MBMS),and is thus not a future proof solution.

According to another approach, currently used for UMTS and discussed forLTE, special periods called idle periods downlink (IPDL) in a cell (cellIPDL) or site (site IPDL) may be used for PRS transmission. Notransmission occurs during the IPDL. This approach has been used in UMTSnetworks, and due to the radio technology specifics it has only beenconsidered for the entire system bandwidth. Using IPDL for the entiresystem bandwidth may result in inefficient resource utilization fortechnologies that admit larger system bandwidths and allow fortransmissions over smaller parts of the bandwidth.

SUMMARY

Embodiments of the present invention advantageously enable RSmeasurements together with a configurable RS transmission bandwidth. Thepresent invention allows for better interference coordination of RS andmore efficient radio resource utilization, which in turn improves the UERS measurements and thus also the different services using RSmeasurements, such as positioning.

In one embodiment of the present invention, a method implemented by aradio base station for reference signal, RS, measurements in an OFDMsystem is provided. The radio base station is configured to transmit toa user equipment over a defined system bandwidth. The method includesretrieving a RS transmission bandwidth for a cell, where the RStransmission bandwidth is smaller than the system bandwidth. The methodalso includes determining a RS measurement bandwidth based on the RStransmission bandwidth, and transmitting the RS measurement bandwidth toa user equipment when the RS measurement bandwidth is smaller than theRS transmission bandwidth, in order for the user equipment to measurethe RS of the cell.

In another embodiment of the present invention, a method implemented bya user equipment for reference signal, RS, measurements in an OFDMsystem is provided. The method includes receiving an RS measurementbandwidth for a cell, where the RS measurement bandwidth is smaller thanthe system bandwidth, and measuring the RS of the cell in a bandwidthdetermined by the received RS measurement bandwidth and the userequipment capability.

In yet another embodiment of the present invention, a radio base stationfor an OFDM system is provided. The radio base station comprises aretrieving circuit for retrieving a RS transmission bandwidth for acell, where the RS transmission bandwidth is smaller than the systembandwidth. It also comprises a determining circuit for determining a RSmeasurement bandwidth for the cell based on the RS transmissionbandwidth, and a transmitter for transmitting the RS measurementbandwidth to a user equipment when the RS measurement bandwidth issmaller than the RS transmission bandwidth, in order for the userequipment to measure the RS of the cell.

In still another embodiment of the present invention, a user equipmentfor an OFDM system is provided. The user equipment comprises a receiverfor receiving an RS measurement bandwidth for a cell, where the RSmeasurement bandwidth is smaller than the system bandwidth. It alsocomprises a measuring circuit for measuring the RS of the cell in abandwidth determined by the received RS measurement bandwidth and theuser equipment capability.

These and other embodiments of the present invention advantageouslyprovide a flexible configuration of the RS bandwidth, which makes itpossible to do more advanced interference coordination, resulting inlower interference and thus improved RS measurements. Embodiments of thepresent invention also measure over only a part of the system, whichallows for a reduced UE complexity.

Still further, embodiments of the present invention enable moreefficient utilization of LIS resources. Different services or featuresof LTE and its extensions may require using different LIS and referencesignals. Increasing the number of LIS may be not desirable from thenetwork performance point of view, due to the wasted capacity. Definingnew specific reference signals may also be difficult in the standard.Therefore, if the measurement granularity is sufficient with a smallerbandwidth, it is more efficient to allocate the rest of the bandwidth inthe same sub frame for other transmissions.

Embodiments of the present invention also give a more efficient spectrumutilization and minimize capacity loss. Yet these embodiments stillenable suppression of data transmissions in sub frames, to providelow-interference conditions, as data transmissions are suppressed onlyin a part of the system bandwidth. This is possible due to the flexiblelow interference bandwidth solution.

Embodiments of the present invention further offer system flexibility.Not all of the network components transmitting reference signals arenecessarily of the same type. For example, the network may have amulti-layer structure consisting of macro and micro RBS. There can alsobe other assisting devices, like for example devices with only a limitedRBS functionality. With such a variety of network components, there isno guarantee that they all can transmit over the same bandwidth.

In addition, embodiments of the present invention address spectrumissues, e.g. by limiting transmissions of reference signals to a certainpart of the bandwidth in low-interference sub frames. With thepossibility of power boosting, which has been discussed in the 3GPPstandardization meetings, and the flexibility in power spectral densitythat may lead to a higher variability in the emitted power, it maybecome challenging to meet all the power emission requirements (e.g.limited out-of band power emission). The flexible bandwidth allocationof embodiments of the present invention allows for a more flexibleconfiguration, which may be helpful in addressing similar issues.

Embodiments of the present invention also provide efficient interferencecoordination for RS, due to flexibility in cell grouping and bandwidthallocation for the cell groups. Given that the allocated bandwidthprovides a sufficient measurement granularity, multiplexing several cellgroups in the same sub frame gives a gain similar to what could beobtained with a higher frequency reuse. Assigning the strongestinterferers to different groups is likely to result in significantlyimproved measurements. Furthermore, the gain may be enough to avoidintroducing IPDL into LTE, which will significantly simplify the designof the involved network elements.

Of course, the present invention is not limited to the above featuresand advantages. Indeed, those skilled in the art will recognizeadditional features and advantages upon reading the following detaileddescription, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a part of a conventional LTE systemwherein the present invention may be implemented.

FIGS. 2a-b illustrate schematically synchronization signals in LTE.

FIGS. 3a-d illustrate schematically the CRS pattern in a LTEtime-frequency resource grid for different numbers of antenna ports.

FIG. 4 illustrates schematically a low interference sub frame with anexample of a PRS pattern repeated over all PRB.

FIG. 5 illustrates schematically aligned low interference sub framesshowing the frequency range of one PRB for each cell and PRS RE with afrequency reuse factor of three.

FIG. 6 illustrates schematically an example of grouping of cells whereeach group is assigned a certain bandwidth for transmitting PRS.

FIGS. 7a-b are flowcharts of the methods in the RBS according toembodiments of the present invention.

FIGS. 8a-b are flowcharts of the methods in the UE according toembodiments of the present invention.

FIG. 9 illustrates schematically the RBS and the UE according toembodiments of the present invention.

DETAILED DESCRIPTION

In the following, the invention will be described in more detail withreference to certain embodiments and to accompanying drawings. Forpurposes of explanation and not limitation, specific details are setforth, such as particular scenarios, techniques, etc., in order toprovide a thorough understanding of the present invention. However, itwill be apparent to one skilled in the art that the present inventionmay be practiced in other embodiments that depart from these specificdetails.

Moreover, those skilled in the art will appreciate that the functionsand means explained herein below may be implemented using softwarefunctioning in conjunction with a programmed microprocessor or generalpurpose computer, and/or using an application specific integratedcircuit (ASIC). It will also be appreciated that while the currentinvention is primarily described in the form of methods and devices, theinvention may also be embodied in a computer program product as well asin a system comprising a computer processor and a memory coupled to theprocessor, wherein the memory is encoded with one or more programs thatmay perform the functions disclosed herein.

The present invention is described herein by way of reference toparticular example scenarios. In particular the invention is describedin a non-limiting general context in relation to a 3GPP LTE system andto RS (reference signals) for positioning in LTE, i.e. PRS (positioningreference signals). It should though be noted that the invention and itsexemplary embodiments may also be applied to other types of OFDMsystems, such as WiMax and coming releases of LTE, and are not limitedto a specific type of reference signals. The invention and itsembodiment are thus also relevant for other radio access technologieswith a capability of transmitting signals over a smaller part of thesystem bandwidth and utilizing reference or pilot signals transmitted indownlink to facilitate UE measurements, and for all wireless devicesthat perform measurements on reference signals transmitted by thenetwork.

In the present invention, the drawbacks of using the entire systembandwidth for the RS transmission pattern, are addressed by a solutionallowing for an adaptation of the bandwidth in which the RS aretransmitted, which will hereinafter be referred to as flexible RSbandwidth. For a specific UE that needs to measure a RS from a certaincell, the RBS retrieves the RS transmission bandwidth that correspondsto that cell, this RS transmission bandwidth being smaller than thesystem bandwidth. The RBS then determines a RS measurement bandwidthbased on the RS transmission bandwidth (i.e. within the RS transmissionbandwidth), and transmits it to the UE in order for the UE to be able tomeasure the RS. If the RS measurement bandwidth is the same as the RStransmission bandwidth, i.e. if the RBS determines that the measurementshould be done over the entire RS transmission bandwidth, the explicitsignaling of the measurement bandwidth to the UE is not needed, as thethe RS transmission bandwidth is always signaled to the UE. Thecapability of the UE may restrict the RS measurement bandwidth evenfurther, if the UE only allows a measurement over a smaller bandwidththan the RS measurement bandwidth, determined by the RBS, indicates. Theflexible RS bandwidth solution makes it possible to allocate thetransmission bandwidth and the measurement bandwidth for the RS in a waythat minimizes interference, by choosing to transmit and measure inlow-interference PRBs in general. Flexible RS bandwidth may be appliedto RS transmitted in downlink in fully aligned (in synchronous networks)or in partially aligned (in asynchronous networks) sub frames.

To support flexible RS bandwidth, it is necessary to make sure that UEsare informed about which PRBs they have to measure on, as stated above.By default, the UE may assume that the RS measurement bandwidth is theentire RS transmission bandwidth, unless the UE is instructed by thenetwork on the bandwidth to measure. In general the RS transmissionbandwidth is larger than the RS measurement bandwidth the UE isinstructed to measure on.

As mentioned above, the flexible RS bandwidth solution enables the cellsto transmit RSs over a smaller part of the system bandwidth. The RStransmission bandwidth can be configured statically, semi-statically ordynamically. In a first exemplary embodiment of the present invention,the RS transmission bandwidth is statically configured in the system,and may e.g. be decided by the network operator. In more advancedsolutions, the allocation can be decided by a radio resource management(RRM) algorithm that may also require inter-cell coordination. In asecond exemplary embodiment of the present invention, the RStransmission bandwidth is dynamically configured based on a RRMalgorithm in the radio base station, without any coordination withneighboring sites. The RBS may in the algorithm e.g. take theinterference situation of the own cells into account. In a thirdexemplary embodiment of the present invention, the RS transmissionbandwidth is dynamically configured based on an algorithm that requiresinter-cell coordination. The RBS may then coordinate with neighboringRBS, e.g. via the X2 interface between the eNodeBs in LTE. In a fourthexemplary embodiment of the present invention, the RS transmissionbandwidth is determined by a controlling node connected to the RBS, andthe RBS then needs to retrieve the RS transmission bandwidth from thecontrolling node. The controlling node could e.g. be the eSMLC node inLTE.

When configuring the flexible RS bandwidth, it is possible to configuredifferent RS bandwidths to different groups of cells. The cells are thusconfigured to transmit RS only in the PRB associated with the group. Anexample is illustrated in FIG. 6 where five groups are assumed, eachgroup allocated five consecutive PRBs non-overlapping with theother-groups PRBs, i.e. 25 PRBs in total (for a total of 5 MHz systembandwidth). In the example, each group is allocated a contiguous part ofthe system band and the allocated bandwidths do not overlap among thegroups, although neither of the two is a limitation of the presentedidea. In a special case, each group may consist of a single cell. Inanother special case, all cells are in the same group, and the group isallocated a bandwidth smaller than the total system bandwidth. Ingeneral, it is not required that the full bandwidth is allocated, northat all cells or cell groups have allocated PRBs within the same subframe. In one embodiment of the present invention, a cell (or cellgroup) is allocated consecutive PRBs (i.e. a contiguous part of thebandwidth). This is desirable to reduce the UE complexity.

In one embodiment of the present invention, the RS measurement bandwidthis the same for all UE in a cell, i.e. it is cell specific. In analternative embodiment the RS measurement bandwidth is UE specific. A RSmeasurement bandwidth that is UE specific may be beneficial in the caseof a cell with two UE close to the cell border, on opposite sides of thecell. In those parts of the network the strongest interfering cells maynot be the same which means that it may be better to let these UEmeasure the RS in different parts of the RS transmission bandwidth.

A UE specific RS measurement bandwidth could be decided either in thenetwork, e.g. based on interference statistics, and transmitted to theUE as described above. Alternatively, it could be decided by the UE,transparently to the network. The UE will thus decide whether to use theRS measurement bandwidth transmitted by the network or a RS measurementbandwidth determined by its own. The UE may for example determine the RSmeasurement bandwidth based on information about the SI NR per PRB. Atrigger for the UE to decide whether it should reconsider themeasurement bandwidth, could be that the RBS does not transmit any RSmeasurement bandwidth.

In one embodiment of the present invention, the RS measurement bandwidthfor a cell is transmitted to the UE by including it in the assistancedata. The assistance data is conventionally used by the RBS to transmitcell identities of neighboring cells, in order for the UE to know whatcells to measure. The RS measurement bandwidth could e.g. be given inthe format (BW_first, BW_length), where BW_first is the index of thefirst PRB of the RS measurement bandwidth and BW_length is its length interms of the number of PRBs. Another alternative is to use the format(BW_0, BW_offset), where BW_0 is the centre of the bandwidth andBW_offset is half of the actual RS measurement bandwidth. The benefit ofthis format is that it results in a shorter message since the maximumvalue of the second parameter is half as large as that of BW_lengthparameter.

Another possibility for transmitting the RS measurement bandwidth (aswell as the RS transmission bandwidth) is to transmit bitmaps with arelation to cell identities via the assistance data. The bitmap doesimplicitly indicate the RS transmission/measurement bandwidthcorresponding to the related cell. These bitmaps may then be stored bythe UE, and the UE can retrieve the RS transmission/measurementbandwidth based on the identity of the cell that needs to be measured.When the RS measurement bandwidth does not change dynamically, i.e. witha unique mapping between cell identity and bandwidth, this solutionwould make it possible to transmit the bitmaps with a lower frequency,e.g. retransmitted only upon request from the UE or upon changes in thenetwork.

In LTE, the signaling of the RS transmission and measurement bandwidthto the UE can be configured by higher layers and performed over theRadio Resource Control (RRC) protocol or the LTE Positioning Protocol(LPP), and the source of the signaling can be the RBS (eNodeB) or theeSMLC (in this case signalled transparently via the eNodeB), dependingon where the decision about the RS measurement bandwidth is taken. Thesignaling can be broadcast, e.g. as part of system information, or itmay be a dedicated signaling. The dedicated signaling may be the samefor the whole network, if the RS transmission bandwidth is staticallyconfigured and is the same for all cells, e.g. in a 10 MHz system wherethe RS are configured to be transmitted over 5 MHz centered at the DCcarrier in all cells. The signaling may also be cell specific if the RStransmission bandwidth varies from cell to cell.

In some cases, the UE has to re-calculate the RS measurement bandwidthaccording to the rule: UE_measurement_bandwidth=min(system_bandwidth,RBS_measurement_bandwidth, UE_capability_bandwidth), wheresystem_bandwidth is the system bandwidth applicable for the cell the UEis measuring on, RBS_measurement_bandwidth is the sum of all PRBsindicated to the UE to perform the measurements on, andUE_capability_bandwidth is the bandwidth that the UE is capable tomeasure. This may be the case e.g. in a network with a macro and a microcell layer, as the system bandwidth for a micro cell may be smaller thanthe one for the macro cell. In yet another case, the UE may apply thesuggested measurement bandwidth symmetrically around the centerfrequency.

In another embodiment of the present invention, the interferencesituation is even further improved by introducing the possibility tosuppress data transmission in a sub frame, not over the entirebandwidth, as in the LIS, but over a configurable part of the bandwidth.This solution will hereinafter be referred to as a flexible lowinterference bandwidth (LIB). The data may e.g. be suppressed only inthe part of the bandwidth where the RS are transmitted or measured. TheRS transmission bandwidth and the LIB can in general be configuredseparately and irrespectively of each other, and do not necessarilycoincide. The existing LIS definition is a special case with the LIBequal to the entire bandwidth. In another special case, the LIB may alsocoincide with the RS measurement bandwidth. One advantage of flexibleLIB, is that it gives a more efficient spectrum utilization andminimizes capacity loss as data transmissions are suppressed only in apart of the system bandwidth.

LIB is intended to reduce interference on some PRBs in order to improveRS measurements. When the RBS determines the RS measurement bandwidth,it takes into account the LIB of the cell serving the UE as well asneighboring cells LIB, in order for the UE to measure in lowinterference conditions.

Similar to the flexible RS configuration, LIB configuration can beconfigured statically, semi-statically or dynamically and can bedecided, for example, by the network operator or an RRM algorithm anddynamically in coordination with neighboring RBSs. It may also be acontrolling node that determines the LIB configuration. The LIB,however, does not need to be known at the UEs, so the actual LIB doesnot need to be transmitted to the UE. The RS measurement bandwidthtransmitted to the UE will already take into account the LIB. In oneexample, the RS measurement bandwidth transmitted to the UE may includeonly PRBs within the LIB.

In an exemplary embodiment of the present invention the determination ofthe RS measurement bandwidth is also based on interference statisticswhich may already be available in the RBS and used for other purposes.

In one embodiment of the present invention, the reference signals arethe PRS in an LTE system. By using flexible PRS transmission bandwidth,possibly also together with the flexible LIB, the measurement of the PRSused for positioning will be considerably improved through theinterference reduction, thus enabling an accurate positioning service.At the same time, the flexible configuration of the bandwidth will makeit possible to provide accurate positioning with a minimum of spectrumutilization.

FIG. 7a is a flowchart of the method in the RBS according to oneembodiment of the present invention. It comprises the following steps:

-   -   710: Retrieve the RS transmission bandwidth, smaller than the        system bandwidth, for the cell that is to be measured. The RS        transmission bandwidth is either statically configured in the        RBS or it may be dynamically configured, either by a RRM        algorithm in the RBS which may also require a coordination with        neighboring RBS. The RS transmission bandwidth may also be        retrieved from a connected controlling network node, such as the        eSMLC in LTE.    -   720: Determine the RS measurement bandwidth based on the        retrieved RS transmission bandwidth. The RS measurement        bandwidth may be smaller than the RS transmission bandwidth, and        may be allocated to a low interference part of the RS        transmission bandwidth e.g. By default, the RS measurement        bandwidth may be equal to the RS transmission bandwidth. The RS        measurement bandwidth may be cell specific or UE specific.    -   730: Transmit the RS measurement bandwidth to the UE, in order        for the UE to know where to measure the RS. This is only needed        if the RS measurement bandwidth differs from the RS transmission        bandwidth, as default is to measure in the RS transmission        bandwidth. The signaling of the RS measurement bandwidth may be        done e.g. via the assistance data transmitted to the UE.

FIG. 7b is a flowchart of the method in the RBS according to anotherembodiment of the present invention. It comprises the steps illustratedin FIG. 7a described above, preceded by the new step 700 of suppressingthe data transmission in a so called LIB of a sub frame, where the LIBis smaller than the system bandwidth. In this embodiment, the step 720of determining the RS measurement bandwidth will be based, not only onthe RS transmission bandwidth, but also on the LIB of the current andthe neighboring cell(s). The purpose is to allow for a reducedinterference when measuring the RS, e.g. for positioning.

FIG. 8a is a flowchart of a the method in the UE according to oneembodiment of the present invention. It comprises the following steps:

-   -   810: Receive an RS measurement bandwidth for a cell, where the        RS measurement bandwidth is smaller than the system bandwidth.    -   820: Measure the RS in a bandwidth determined by the received RS        measurement bandwidth and by the UE capability. If the RS        measurement bandwidth indicates 15 MHz and the UE capability        only allows for measuring over 10 MHz bandwidth, the RS        measurement bandwidth signaled by the RBS cannot be used. The UE        must then adapt the measurement bandwidth to what is possible.

FIG. 8b is a flowchart of a the method in the UE according to anotherembodiment of the present invention. It comprises the further step ofstoring 815 the RS measurement bandwidth for a cell together with thecell identity, in e.g. a bitmap. The UE may then retrieve the RSmeasurement bandwidth from the stored bitmap based on a cell identity,when it needs to measure the RS for the corresponding cell.

Schematically illustrated in FIG. 9, and according to embodiments of thepresent invention, is the RBS 900. It comprises a retrieving circuit901, for retrieving the RS transmission bandwidth, which is eitherstatically or dynamically configured and is retrieved either from theRBS itself or from a controlling network node connected to the RBS. TheRBS 900 also comprises a determining circuit 902 for determining the RSmeasurement bandwidth, based on the retrieved RS transmission bandwidth,and a transmitter 903 for transmitting the RS measurement bandwidth tothe UE, e.g. via the assistance data, when it differs from the RStransmission bandwidth. According to one embodiment, the RBS alsocomprises a data suppressing circuit 904 for suppressing datatransmissions in a sub frame over a LIB smaller than the systembandwidth. In this embodiment the determining circuit 902 is adapted todetermine the RS measurement bandwidth also based on the LIB of theserving cell and of neighboring cells.

Also illustrated in FIG. 9, and according to embodiments of the presentinvention, is the UE 950. It comprises a receiver 951 for receiving theRS measurement bandwidth, e.g. in the assistance data from the RBS, anda measuring circuit 952 for measuring the RS in the bandwidth determinedby the received RS measurement bandwidth and the UE capability. In oneembodiment of the present invention, the UE 950 also comprises a storingcircuit 953 for storing the RS measurement bandwidth together with thecell identity, in order for the UE to be able to retrieve the RSmeasurement bandwidth when needed.

The above mentioned and described embodiments are only given as examplesand should not be limiting to the present invention. Other solutions,uses, objectives, and functions within the scope of the invention asclaimed in the accompanying patent claims should be apparent for theperson skilled in the art.

ABBREVIATIONS

3GPP 3rd Generation Partnership Project

A-GPS Assisted GPS

BS Base Station

CRS Cell-specific Reference Signal

eNodeB evolved Node B

eSMLC evolved SMLC

GPS Global Positioning System

GSM Global System for Mobile communications

IPDL Idle Period Downlink

LIB Low-Interference Bandwidth

LIS Low-Interference Subframe

LPP LTE Positioning Protocol

LTE Long-Term Evolution

MAC Medium Access Control

MBSFN Multicast Broadcast Single Frequency Network

MBMS Multimedia Broadcast Multicast Service

OFDM Orthogonal Frequency Division Multiplexing

OTD Observed Time Difference

OTDOA Observed Time Difference Of Arrival

PDSCH Physical Downlink Shared Channel

PRB Physical Resource Block

PRS Positioning Reference Signal

PSS Primary SS

RE Resource Element

RRM Radio Resource Management

RS Reference Signal

SINR Signal-to-Interference plus Noise Ratio

SMLC Serving Mobile Location Center

SS Synchronization Signal

SSS Secondary SS

TDOA Time Difference of Arrival

UE User Equipment

UMTS Universal Mobile Telecommunications System

1. A method implemented by a controlling network node for referencesignal, RS, measurements in an OFDM system, the radio base stationconfigured to transmit to a user equipment over a defined systembandwidth, the method comprising: retrieving a RS transmission bandwidthfor a cell, said RS transmission bandwidth being smaller than the systembandwidth, determining a RS measurement bandwidth based on the RStransmission bandwidth, and transmitting the RS measurement bandwidth toa user equipment when said RS measurement bandwidth is smaller than theRS transmission bandwidth, in order for the user equipment to measurethe RS of the cell.